File.open with block (auto-close) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using File.open with a block?

Consider the following Ruby code snippet:

File.open("test.txt", "w") do |file|
  file.puts "Hello, world!"
end
puts File.exist?("test.txt")

What will be printed when this code runs?

Ruby

File.open("test.txt", "w") do |file|
  file.puts "Hello, world!"
end
puts File.exist?("test.txt")

A nil

B true

C false

D An error is raised

💡 Hint

Think about whether the file is created and closed properly after the block.

The File.open method with a block automatically closes the file after the block finishes. The file "test.txt" is created and written to, so File.exist?("test.txt") returns true.

❓ Predict Output

intermediate

What does this code print when reading a file with File.open and a block?

Given a file "data.txt" containing the text "Ruby Rocks!", what will this code output?

File.open("data.txt", "r") do |file|
  puts file.read
end

Ruby

File.open("data.txt", "r") do |file|
  puts file.read
end

A An error is raised

B An empty line

C Ruby Rocks!

D nil

💡 Hint

Remember that file.read reads the entire content of the file.

The block reads the entire content of "data.txt" and prints it. Since the file contains "Ruby Rocks!", that is printed.

❓ Predict Output

advanced

What error does this code raise when using File.open without a block but forgetting to close?

What happens when you run this Ruby code?

file = File.open("example.txt", "w")
file.puts "Hello"
# No file.close called

Ruby

file = File.open("example.txt", "w")
file.puts "Hello"

A IOError: not closed

B RuntimeError: closed stream

C SyntaxError

D No error, but file remains open until program ends

💡 Hint

Think about what happens if you don't explicitly close a file in Ruby.

If you open a file without a block and don't close it, Ruby does not raise an error immediately. The file stays open until the program ends or the object is garbage collected. This can cause resource leaks but no immediate error.

🧠 Conceptual

advanced

Why is using File.open with a block preferred over without a block?

Which of the following is the main advantage of using File.open with a block?

A It automatically closes the file after the block finishes

B It speeds up file reading

C It prevents any errors from occurring

D It makes the file read-only

💡 Hint

Think about resource management and safety.

Using File.open with a block ensures the file is closed automatically after the block ends, preventing resource leaks and errors from forgetting to close the file.

❓ Predict Output

expert

What is the output of this Ruby code using File.open with a block and exception handling?

Consider this Ruby code:

begin
  File.open("missing.txt", "r") do |file|
    puts file.read
  end
rescue Errno::ENOENT
  puts "File not found"
end

What will be printed when this code runs and the file "missing.txt" does not exist?

Ruby

begin
  File.open("missing.txt", "r") do |file|
    puts file.read
  end
rescue Errno::ENOENT
  puts "File not found"
end

A File not found

B nil

C An unhandled exception is raised

D An empty line

💡 Hint

Think about how exceptions are handled in Ruby.

The code tries to open a file that does not exist. This raises Errno::ENOENT, which is rescued and prints "File not found".

Practice

1. What happens to the file after the block finishes when you use File.open('file.txt') do |f| ... end in Ruby?

easy

A. The file remains open until you manually close it.

B. The file is automatically closed after the block ends.

C. The file is deleted after the block ends.

D. The file is locked and cannot be accessed again.

Solution

1. Step 1: Understand File.open with block behavior

   When you open a file with a block, Ruby automatically manages the file resource.

2. Step 2: Recognize automatic closing

   After the block finishes, Ruby closes the file automatically to prevent resource leaks.

3. Final Answer:

   The file is automatically closed after the block ends. -> Option B

4. Quick Check:

   File.open with block = auto-close [OK]

Hint: File.open with block always auto-closes the file [OK]

Common Mistakes:

• Thinking the file stays open after the block
• Assuming the file is deleted automatically
• Believing the file locks after block ends

2. Which of the following is the correct syntax to open a file named data.txt for reading using a block in Ruby?

easy

A. File.open('data.txt') |file| puts file.read end

B. File.open('data.txt') puts file.read end

C. File.open('data.txt') { |file| puts file.read }

D. File.open('data.txt') do puts file.read end

Solution

1. Step 1: Identify correct block syntax

   Ruby uses either curly braces or do...end with a block parameter like |file|.

2. Step 2: Check each option

   File.open('data.txt') { |file| puts file.read } correctly uses { |file| ... }. Others have syntax errors or missing block parameters.

3. Final Answer:

   File.open('data.txt') { |file| puts file.read } -> Option C

4. Quick Check:

   Correct block syntax = File.open('data.txt') { |file| puts file.read } [OK]

Hint: Use {|var| ...} or do |var| ... end for blocks [OK]

Common Mistakes:

• Missing block parameter |file|
• Incorrect block delimiters
• Omitting do or braces

3. What will be the output of this Ruby code?

File.open('test.txt', 'w') do |f|
  f.puts 'Hello'
end

content = File.read('test.txt')
puts content

medium

A. Hello

B. test.txt

C. nil

D. Error: file not found

Solution

1. Step 1: Write 'Hello' to file inside block

   The block writes the string 'Hello' followed by a newline to 'test.txt'.

2. Step 2: Read and print file content

   After the block, the file is closed automatically. Then File.read reads the content, which is 'Hello\n'. Printing it outputs 'Hello'.

3. Final Answer:

   Hello -> Option A

4. Quick Check:

   File.write + read = Hello [OK]

Hint: File.open with block writes and auto-closes file [OK]

Common Mistakes:

• Expecting file to be empty
• Thinking file is not saved before reading
• Confusing file name with content

4. Identify the error in this Ruby code snippet:

file = File.open('log.txt', 'w') do |f|
  f.puts 'Log entry'
end
file.puts 'Another entry'

medium

A. file is nil because File.open with block returns the block's value, not the file object.

B. The file is not opened in write mode.

C. Missing block parameter |f| in File.open.

D. File is not closed automatically.

Solution

1. Step 1: Understand return value of File.open with block

   When using a block, File.open returns the block's last expression, not the file object.

2. Step 2: Analyze variable 'file'

   Here, 'file' is assigned the block's return value, which is nil because f.puts returns nil. So calling file.puts causes an error.

3. Final Answer:

   file is nil because File.open with block returns the block's value, not the file object. -> Option A

4. Quick Check:

   File.open block returns block value, not file [OK]

Hint: File.open with block returns block result, not file object [OK]

Common Mistakes:

• Expecting File.open with block to return file object
• Trying to use file after block ends
• Ignoring file mode argument

5. You want to read a file and count how many lines contain the word 'error'. Which Ruby code correctly uses File.open with a block to do this safely?

hard

A. count = 0
File.open('log.txt') { |f| f.each { |line| count += 1 if line.include?('error') } }
puts count

B. count = 0
f = File.open('log.txt', 'r')
f.each_line { |line| count += 1 if line.include?('error') }
f.close
puts count

C. count = 0
File.open('log.txt', 'r') do
each_line { |line| count += 1 if line.include?('error') }
end
puts count

D. count = 0
File.open('log.txt', 'r') do |f|
f.each_line { |line| count += 1 if line.include?('error') }
end
puts count

Solution

1. Step 1: Use File.open with block and read lines

   count = 0 File.open('log.txt', 'r') do |f| f.each_line { |line| count += 1 if line.include?('error') } end puts count opens the file with a block and iterates each line safely.

2. Step 2: Count lines containing 'error'

   Inside the block, it increments count if the line includes 'error'. The file auto-closes after the block.

3. Final Answer:

   count = 0 File.open('log.txt', 'r') do |f| f.each_line { |line| count += 1 if line.include?('error') } end puts count -> Option D

4. Quick Check:

   File.open block + each_line + count = count = 0 File.open('log.txt', 'r') do |f| f.each_line { |line| count += 1 if line.include?('error') } end puts count [OK]

Hint: Use File.open with block and each_line for safe file reading [OK]

Common Mistakes:

• Forgetting to close file when not using block
• Missing block parameter or using wrong syntax
• Using methods outside block scope